Solid particles entrained in a moving fluid within a conduit can be monitored using acoustic means. Specifically, the sonic and ultrasonic signatures caused when the particles impact the wall of the conduit can be detected due to the transfer of momentum to the surrounding conduit. A sensor such as an acoustic transducer can be inserted either directly into the fluid flow or attached to the surrounding conduit. The fluid can be either a gas such as air or a viscous fluid such and oil or water. Particle impacts occur when the motion of the fluid is changed by a modification of the flow such as a by a bend in the conduit.
U.S. Pat. No. 5,969,236 discloses a particle sensor comprising a sensor element including a vibrating section having a mass that is sensitive to the collision of solid particles contained in a fluid and a detecting section for detecting the vibration of the vibrating section caused by the collision and converting the vibration to electric signals. The particle sensor also includes through holes that are formed in the sensor element in the vicinity of the vibrating section, such that the fluid entering the particle sensor from an inlet nozzle collides with the sensor element, passes through the through holes and leaves the particle sensor from the outlet nozzle. The particle sensor is adapted to measure the sizes and particle size distribution of particles contained in the fluid.
The particle sensor described in U.S. Pat. No. 5,969,236 suffers from a number of drawbacks. One drawback is that means are not provided to ensure that the particles impact the particle sensor in a direction substantially normal to the front face of the sensor rather than following the fluid flow away from the sensor. A further drawback is that the electrical signal produced by impinging particles is not necessarily a monotonic function of the momentum of the impinging particles. An additional drawback is that this reference does not provide an indicator for indicating when integrated particle impacts are greater than an acceptable threshold revel.
In view of the above, there exists a need for an acoustic particle alarm having a particle sensor comprising a monitoring flow cell including an acoustic particle sensor that produces an output signal in response to particle (or contaminant) impacts in a fluid flow that is impinging on the acoustic sensor.
There also exists a need for an acoustic particle alarm having a particle sensor comprising a monitoring flow cell including an acoustic sensor that produces an output signal in response to particle impacts in a fluid flow that is impinging on the acoustic sensor, wherein the acoustic sensor is isolated from the flow cell to reduce the acoustic coupling between the sensor and the other elements of the flow cell.
There further exists a need for a method of establishing the fluid flow onto an acoustic sensor such that particulate matter in fluid flow impacts a face of the acoustic sensor in a direction substantially normal to the face of the acoustic sensor.
Additionally, there exists a need for a method of establishing fluid flow onto an acoustic sensor such that particulate matter in fluid flow impacts a face of the acoustic sensor with as much velocity as is permitted by flow noise.
There also exists a need for a method of establishing fluid flow onto an acoustic sensor such that particulate matter in fluid flow impacts a face of the acoustic sensor rather than following the fluid flow away from the sensor.
There further exists a need for a method of establishing fluid flow onto an acoustic sensor at a preferred flow velocity.
Additionally, there exists a need for a method of establishing fluid flow onto an acoustic sensor, wherein the particle impact signal is separable from the flow noise.
There also exists a need for a method of establishing fluid flow onto an acoustic sensor, wherein the amplitude of the electrical signal produced by impinging particles is monotonic in the momentum of the impinging particles.
There further exists a need for a method of establishing fluid flow onto an acoustic sensor and measuring the integrated contribution of impinging particles on the acoustic sensor.
Additionally, there exists a need for an acoustic particle alarm including a particle sensor that produces an output signal in response to particle impacts in a fluid flow and an indicator for indicating when the integrated particle impacts are greater than an acceptable threshold level.